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Title: Subdominant modes and optimization trends of DIII-D reverse magnetic shear configurations

Abstract

Alfvén Eigenmodes (AE) and magneto-hydrodynamic (MHD) modes are destabilized in DIII-D reverse magnetic shear configurations and may limit the performance of the device. We use the reduced MHD equations in a full 3D system, coupled with equations of density and parallel velocity moments for the energetic particles as well as the geodesic acoustic wave dynamics, to study the properties of instabilities observed in DIII-D reverse magnetic shear discharges, with the purpose of discovering ways to avoid or minimize MHD and AE activity for different magnetic field configurations and neutral beam injector (NBI) operation regimes. The simulations show at the beginning of the discharge, before the reverse shear region is formed, a plasma AE unstable and marginally MHD stable. As soon as the reverse shear region appears, ideal MHD modes are destabilized with larger growth rate than the AE. Both MHD modes and AE coexist along the discharge, although the MHD modes are more unstable as the reverse shear region deepens. AE stability can be improved if the NBI injection is off axis, because on-axis injection leads to AE with larger growth rate and frequency. In addition, decreasing the beam energy or increasing the NBI injection lead to an AE withmore » larger growth rate and frequency, so a NBI operation in the weakly resonant regime requires higher beam energies than in the experiment (V th,ƒ/V A0 > 0.3). The MHD linear stability can be also improved if the reverse shear region and the q profile near the magnetic axis are in between the rational surfaces q = 2 and q = 1, specifically if there is a region in the core with negative shear, avoiding a flat q profile near the magnetic axis. The simulations also shows a smooth transition between MHD modes and low frequency AE, no critical β ƒ, pointing out an overlap between MHD and AE activity for modes with frequency lower than 30 kHz. This is in the range of Beta Acosutic Alfvén Eigenmodes (BAAE) and Beta induced Alfvén Eigenmodes (BAE).« less

Authors:
ORCiD logo [1]; ORCiD logo [1];  [1];  [2];  [1];  [1]; ORCiD logo [1]
  1. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
  2. Univ. Carlos III de Madrid (Spain)
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States); General Atomics, San Diego, CA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Fusion Energy Sciences (FES)
OSTI Identifier:
1648968
Alternate Identifier(s):
OSTI ID: 1567774
Grant/Contract Number:  
AC05-00OR22725; FC02-04ER54698
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Nuclear Fusion
Additional Journal Information:
Journal Volume: 59; Journal Issue: 4; Journal ID: ISSN 0029-5515
Publisher:
IOP Science
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; Tokamak; DIII-D; pedestal; MHD; AE; energetic particles

Citation Formats

Varela, J., Spong, D. A., Murakami, M., Garcia, L., D’Azevedo, E., Van Zeeland, M. A., and Munaretto, S. Subdominant modes and optimization trends of DIII-D reverse magnetic shear configurations. United States: N. p., 2019. Web. doi:10.1088/1741-4326/ab0052.
Varela, J., Spong, D. A., Murakami, M., Garcia, L., D’Azevedo, E., Van Zeeland, M. A., & Munaretto, S. Subdominant modes and optimization trends of DIII-D reverse magnetic shear configurations. United States. https://doi.org/10.1088/1741-4326/ab0052
Varela, J., Spong, D. A., Murakami, M., Garcia, L., D’Azevedo, E., Van Zeeland, M. A., and Munaretto, S. Wed . "Subdominant modes and optimization trends of DIII-D reverse magnetic shear configurations". United States. https://doi.org/10.1088/1741-4326/ab0052. https://www.osti.gov/servlets/purl/1648968.
@article{osti_1648968,
title = {Subdominant modes and optimization trends of DIII-D reverse magnetic shear configurations},
author = {Varela, J. and Spong, D. A. and Murakami, M. and Garcia, L. and D’Azevedo, E. and Van Zeeland, M. A. and Munaretto, S.},
abstractNote = {Alfvén Eigenmodes (AE) and magneto-hydrodynamic (MHD) modes are destabilized in DIII-D reverse magnetic shear configurations and may limit the performance of the device. We use the reduced MHD equations in a full 3D system, coupled with equations of density and parallel velocity moments for the energetic particles as well as the geodesic acoustic wave dynamics, to study the properties of instabilities observed in DIII-D reverse magnetic shear discharges, with the purpose of discovering ways to avoid or minimize MHD and AE activity for different magnetic field configurations and neutral beam injector (NBI) operation regimes. The simulations show at the beginning of the discharge, before the reverse shear region is formed, a plasma AE unstable and marginally MHD stable. As soon as the reverse shear region appears, ideal MHD modes are destabilized with larger growth rate than the AE. Both MHD modes and AE coexist along the discharge, although the MHD modes are more unstable as the reverse shear region deepens. AE stability can be improved if the NBI injection is off axis, because on-axis injection leads to AE with larger growth rate and frequency. In addition, decreasing the beam energy or increasing the NBI injection lead to an AE with larger growth rate and frequency, so a NBI operation in the weakly resonant regime requires higher beam energies than in the experiment (Vth,ƒ/VA0 > 0.3). The MHD linear stability can be also improved if the reverse shear region and the q profile near the magnetic axis are in between the rational surfaces q = 2 and q = 1, specifically if there is a region in the core with negative shear, avoiding a flat q profile near the magnetic axis. The simulations also shows a smooth transition between MHD modes and low frequency AE, no critical βƒ, pointing out an overlap between MHD and AE activity for modes with frequency lower than 30 kHz. This is in the range of Beta Acosutic Alfvén Eigenmodes (BAAE) and Beta induced Alfvén Eigenmodes (BAE).},
doi = {10.1088/1741-4326/ab0052},
url = {https://www.osti.gov/biblio/1648968}, journal = {Nuclear Fusion},
issn = {0029-5515},
number = 4,
volume = 59,
place = {United States},
year = {2019},
month = {2}
}

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